There are presently a number of techniques for making such shaped layers from plastic material.
For instance, a generally-used technique is heat-shaping, in which a thermoplastic material sheet is heated above the softening temperature thereof and is thereafter distorted inside a mould, corresponding to the negative impression of the desired pattern, either under vacuum action, or by mechanical means, or a combination of both means. This relatively simple technique has however for drawback to be usable in principle but for manufacturing shaped casings or coverings with a not very complex form and a uniform texture. Moreover this technique is limited to the use of plastic materials with a relatively low softening point, in such a way that when said shaped casings or coverings undergo thereafter heating to a temperature about 120.degree. C., the memory effect has a chance to cause distorting of said casings and coverings, which results for example in loosening from the support they are applied on.
Another generally-used technique is the one known as "Slush-moulding" and which comprises filling completely the cavities from a tray-like mould with a liquid synthetic material, such as PVC plastisols.
When contacting the hot mould walls, said material solidifies by forming a film on said walls. The mould is then turned over to remove the excess still liquid synthetic material and cooled to allow stripping the resulting skin. This is usually a rather cumbersome technique which does moreover consume a lot of energy and which subjects the moulds to damaging heat impacts.
A third technique is gun-spraying which comprises spraying a liquid plastic material, diluted or not in solvents, or liquid components which will form by reacting in situ, a polymer, over a mould surface to form thereby thereon a solidified plastic material film.
Said latter technique usually allows to obviate some of the drawbacks from both above-described techniques, but can however be applied only when all the portions from the surface to be coated with the film are easily accessed by the spray from the gun being used to project the liquid plastic material. Thus it is known, by making use of a flexible mould secured with the circumference thereof to a rigid holder the inner shape of which corresponds to the outer shape of the flexible mould, to turn said flexible mould inside out to make the inner surface thereof easily accessed by the gun spray to form thereby over said surface, a flexible plastic material layer, and to then return said mould to the original position thereof which corresponds to the shape of the rigid holder, which is the shape desired for said layer.
Such a technique which appears at first very simple, is not however generally suitable for flexible moulds having a counter-taper, as in such a case it will be substantially impossible to return the mould to the original position thereof and thus to obtain a flexible layer from flexible material having accurately the required shape. Moreover due to the continuous turning inside-out and righting-up of said flexible mould, this one will wear out very fast and will thus not allow reproducing in the plastic material layer the visible surface texture with the required accuracy. Consequently this technique is not generally suitable for manufacturing casings or coverings with narrow and deep cavities, as it is for example the case in some fittings for automotive vehicle dashboards.
One of the essential objects of this invention lies in providing a very simple method which allows obviating this gap in the gun-spraying technique and which is usable without any problem, for preparing shaped layers from plastic material according to a pattern having substantially any shape.